1. Field of the Invention
This invention relates to a leanness sensor for detecting the oxygen concentration in a gas such as the exhaust gas from an internal combustion engine.
2. Description of the Prior Art:
Hitherto, there have been proposed various types of oxygen sensors capable of sensing the oxygen concentration in the exhaust gas of internal combustion engine. On oxygen sensor called a "leanness sensor", which has been developed as one of these known sensors, can detect the oxygen concentration in the exhaust gas over a wide range from low concentration up to a high concentration with accuracy in an order of several tenths of a percent.
The leanness sensor has a disc-shaped element made of a solid electrolyte permeable to oxygen ions, having metallic electrodes formed on both sides thereof. The operation of this leanness sensor is based upon oxygen permeating from one electrode (cathode) to the other electrode (anode) when a voltage is applied between these two electrodes. That is, a threshold current is generated in the element and limited by the flow rate of oxygen from the cathode, and the oxygen concentration in the exhaust gas is measured by continuously detecting the change of amplitude of the threshold current.
FIG. 1 shows the construction of a known leanness sensor. The leanness sensor has an element a including a disc-shaped main body a.sub.1 made of zirconia stabilized by yttrium oxide or the like and electrodes a.sub.2 and a.sub.3, made of platinum or the like material, formed on both sides of the disc-shaped main body a.sub.1. Lead wires b.sub.1 and b.sub.2 made of platinum or the like material are connected at their one ends to the electrodes a.sub.2 and a.sub.3. Porous protective layers a.sub.4 made of alumina, spinel or the like material are formed on both sides of the element a. In order to limit the rate of transmission of oxygen, the electrode covering portions of the protective layers a.sub.4 have different thicknesses on either side of the electrode in order to limit the rate of permeation by oxygen. The lead wires b.sub.1 and b.sub.2 extend through two bores c.sub.1 and c.sub.2 formed in an insulator tube c made from alumina or the like, and are connected to electric wires d.sub.1 and d.sub.2, so that the element a is fixed to the end of the insulator tube c by the lead wires b.sub.1 and b.sub.2. The element a is surrounded by a heater e made of nichrome wire or the like. In operation, the heater e is supplied with electric power to heat the element a up to the operating temperature.
The leanness sensor having the described construction, however, is unstable from a structural point of view because the sensor element a is fixed to and projects from the insulator tube c by means of lead wires b.sub.1 and b.sub.2. The lead wires b.sub.1 and b.sub.2 are usually made of thin platinum wires of a diameter less than 1 mm and therefore lack mechanical strength. In consequence, the lead wires b.sub.1 and b.sub.2 tend to become cut, causing various problems such as breakdown of the element a, detection failure for the element a and so forth.
Usually, the lead wires b.sub.1 and b.sub.2 are made of noble metals such as platinum. This inevitably raises the price of the leanness sensor.